This invention relates to the preparation of diaryl carbonates by carbonylation. More particularly, it relates to the improvement of diaryl carbonate yield in the carbonylation reaction.
Diaryl carbonates are valuable intermediates for the preparation of polycarbonates by transesterification with bisphenols in the melt. This method of polycarbonate preparation has environmental advantages over methods which employ phosgene, a toxic gas, as a reagent and environmentally detrimental chlorinated aliphatic hydrocarbons such as methylene chloride as solvents.
Various methods for the preparation of diaryl carbonates by an oxidative carbonylation (hereinafter sometimes simply "carbonylation" for brevity) reaction of hydroxyaromatic compounds with carbon monoxide and oxygen have been disclosed. In general, the carbonylation reaction requires a rather complex catalyst. Reference is made, for example, to U.S. Pat. No. 4,187,242, in which the catalyst is a heavy Group VIII metal; i.e., a Group VIII metal having an atomic number of at least 44, said metals consisting of ruthenium, rhodium, palladium, osmium, iridium and platinum, or a complex thereof. Palladium catalysts have been found particularly useful; they include complexes with phosphines such as triphenylphosphine.
The production of carbonates may frequently be improved by including a metal-based cocatalyst along with the heavy Group VIII metal catalyst. Suitable metal-based cocatalysts have been described broadly in U.S. Pat. Nos. 4,187,242, 4,201,721 and 5,380,907 as compounds or complexes of copper, iron, manganese, cobalt, mercury, lead, cerium, vanadium, uranium, bismuth and chromium. Cerium compounds as cocatalysts are particularly detailed in European patent application 736,512.
Also required in general as part of the catalyst package are halides, typically quaternary ammonium or phosphonium halides, hexaalkylguanidinium halides or alkali metal or alkaline earth metal halides. According to Japanese Kokai 1/165,551 and 8/134,022, the halide can be an iodide; however, product recovery, particularly as defined in terms of "turnover number" (the number of moles of product and specifically diaryl carbonate formed per gram-atom of palladium present) was calculated to be very low, the highest being represented by a turnover number of about 3.3. In the aforementioned European application 736,512, the halide is a chloride or bromide.
Another disadvantage often encountered in the production of diaryl carbonates is the co-production of isomeric compounds. Thus, diphenyl carbonate yields are often decreased by concurrent formation of isomers such as phenyl salicylate and 4-hydroxyphenyl benzoate.
It remains of interest, therefore, to develop catalyst systems which provide improved production of diaryl carbonates. In particular, it is of interest to increase the rate of production relative to amount of catalyst employed and to decrease the proportions of isomers obtained.